Three principal factors limit the efficiency of radiation emitting diodes. These factors are: non-radiative components of current, absorption of the generated radiation in the semiconductor material before emission, and total internal reflection at the semiconductor-air interface. The last two effects can be minimized separately but it is different to minimize both simultaneously.
The total diode current consists of several parallel components. Each current component depends on the applied junction voltage V as exp [(qV-E)/nkT] where q is the electron charge, E is the bandgap energy, k is Boltzman constant, T is the temperature, and n is a constant. Radiation in infrared emitting diodes results from the radiative recombination of electrons injected across the p-n junction from the N-type material into the P-type material, for the radiative component of current, n = 1. In parallel are non-radiative components of current (n = 2) due to recombination in the space charge region or due to surface recombination. By the use of high quality semiconductor material, the space charge recombination current can be minimized. The principal non-radiative component of current is due to surface recombination.
The result of the non-radiative component of current is non-linearity of the radiant power versus current characteristic. The efficiency of the emitter decreases rapidly at low currents. The degradation of radiant output power versus time at constant current is principally due to an increase in the non-radiative component of current.
One of the principal features of the invention is the use of a multi-layer structure in which the material at the surface has a higher bandgap energy than the material in the interior of the device. One effect of this feature is to greatly reduce the non-radiative component of current due to surface recombination and results in improved linearity and reduced degradation. An application in which the improved emitter linearity results in improved systems performance due to decreased distortion is the analog transmission of information in which the intensity of the emitted radiation is modulated by varying the diode current.
The geometric design of infrared emitting diodes is very important for maximum device efficiency. The high index of refraction of GaAs (n.sub.s = 3.6) compared to that of air (n.sub.a = 1.0) results in a large refraction of rays at the semiconductor-air interface for rays which are not normal to the interface. For flat geometry devices, this large index mismatch severely limits the amount of light which can be obtained from the device.
The basic law of optics which governs the coupling of rays between media of different indices is Snell's law of refraction given by EQU n.sub.a sin.theta..sub.a = n.sub.s sin.theta..sub.s,
Where .theta. is the angle measured from the normal to the interface. The angle .theta..sub.s is the incident angle in the semiconductor. The angle .theta..sub.a is the refracted angle in the air. For small incident angles, the rays will be refracted at the interface. For angles greater than critical angle (.theta..sub.s).sub.c, the total internal reflection of the rays occur. The critical angle for this case is given by EQU sin (.theta..sub.s).sub.c = n.sub.a /n.sub. s
For n.sub.a = 1.0 and n.sub.s = 3.6, the critical angle is equal to 16.1.degree.. Thus only rays within an emission cone with a half angle of 16.1.degree.= can be emitted through the top of a flat emitter.
For spontaneous emitters, the generated radiation in the device is isotropic. The radiant intensity (W/steradian) in the device, I.sub.s, is given by the total generated power P.sub.g divided by the total solid angle. Thus, EQU I.sub.s = P.sub.g /4.pi.
The normal radiant intensity outside a flat geometry emitter is given by EQU (I.sub.a).sub.n = I.sub.s (n.sub.a /n.sub. s) .sup.2
Thus, the radiant intensity normal to the flat device is decreased by a factor of 13 compared to the value inside the device. Also, less than 2% of the total generated radiation can be emitted directly from the top surface of a flat geometry emitter, assuming no reflection from the back contact.
Shaped emitters can be used to eliminate total internal reflections. All rays are incident approximately normal to the semiconductor-air interface for a hemispherical emitter with a small junction diameter. For a hemispherical emitter with no reflection from the back contact, ideally 50% of the generated radiation could be emitted from the device. The normal radiant intensity would be a factor of 13 higher than for the flat geometry emitter.
A truncated spherical (Weierstrass) emitter can be used to actually take advantage of the high refraction at the semiconductor-air interface to help focus the rays along the axis normal to the device, thereby greatly increasing the radiant intensity. Ideally, the average radiant intensity of the truncated spherical emitter can be ten times that of the hemisphere or 130 times that of the flat emitter. However, the finite junction size limits the improvement to about four to seven times improvement compared to the hemispherical emitter. One of the features of the invention is to enable the use of smaller junction diameters and to maximize the amount of power generated near the principal axis of the shaped emitter. These features will maximize the advantages of using the truncated spherical emitter to help collimate the emitted radiation and to increase the radiant intensity.
Another major factor which limits device efficiency is absorption of the generated radiation before being emitted from the device. The spectrum of the generated radiation in the GaAs emitter is near the absorption edge of the GaAs material. Thus, much of high energy radiation is normally absorbed in the GaAs material. The absorption in shaped emitters due to the large path lengths causes the shaped emitters to exhibit less than theoretical improvements compared to the flat emitters. However, the gain achieved with the hemispherical shape exceeds the loss due to absorption, thus resulting in an overall net increase in total output power and normal radiant intensity.
For some applications such as for coupling to small diameter single optical fibers, the total power output and normal radiant intensity is not as important as the value of the radiance. The radiance can be maximized by decreasing the absorption. For small-area, high-radiance emitters, the device is generally mounted P-side down for decreased thermal resistance. The radiation is then emitted through the N-type substrate. One approach to reduce absorption is to etch a well in the back of a flat geometry emitter to greatly reduce the thickness of the GaAs through which the radiation is transmitted. This device design is called the etched-well emitter. Several features of the invention can give improved performance for this type emitter.